Chess Opening Theory/1. e4/1...c5/2. Nf3/2...d6/3. d4/3...cxd4/4. Nxd4/4...Nf6

With 4...Nf6, Black starts the development of his pieces, whilst also attacking the e4 pawn. When Black played 2...d6, he controlled the e5 square, thus preventing White from playing e5 in the future and kicking away this knight. As the e4 pawn is hanging, White does not have time to play 5. c4?, setting up the Maroczy Bind, as 5...Nxe4 would follow, in which White is down a pawn for no compensation.

As a result, White must find a way to defend against this threat.

The main move by far is 5. Nc3, continuing development whilst also defending the e4 pawn. This move also gets White closer to queenside castling, whilst also eyeing the key d5-square, which can be useful if Black plays ...e5 at a later point, where this knight could be excellently placed on d5. In addition, the knight also helps with a king castled on the queenside, as it helps defend against threats on the c-file.

White could also defend the pawn with 5. Bd3, though d3 is often not the best square for this bishop, and this also obstructs the queen's protection of the d4-knight, therefore if Black plays ...Nc6, White will have to spend a tempo defending the knight.

If White insists on creating the Maroczy Bind, he could try the move 5. f3, the Prins Variation, which defends the pawn without blocking the c-pawn, thus giving the option of White to play c4 and set up the Maroczy Bind. As a result, this move discourages moves like 5...a6, 5...g6, and 5...Nc6, as White could then follow up with 6. c4, getting a much improved version of the Najdorf, Dragon, and Classical Sicilians respectively, as Black's counterplay will be limited. As a result, Black usually follows up with 5...e5, aiming to take advantage of White not defending the pawn with a knight, and wasting time with pawn moves.

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6

Wikipedia has related information at Sicilian Defence.